efi_loader: Correctly account the SCRTM event creation

The result of efi_append_scrtm_version() is overwritten before anyone
checks its result. Check it and exit the function on failures

Addresses-Coverity-ID: 467399 Code maintainability issues (UNUSED_VALUE)
Fixes: commit 97707f12 ("tpm: Support boot measurements")
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>